Merge bitcoin/bitcoin#31537: qa: Limit -maxconnections in tests

d9d5bc2 qa: Limit `-maxconnections` in tests (Hennadii Stepanov)

Pull request description:

  On systems such as NetBSD, the `bitcoind` typically prints the following warning:
  ```
  Warning: Reducing -maxconnections from 125 to 96, because of system limitations.
  ```

  This breaks the functional test framework (see bitcoin/bitcoin#23968).

  This PR limits the `-maxconnections` to mitigate the issue and enable functional tests on NetBSD.

  Fixes bitcoin/bitcoin#23968.

  Here is CI log from the [`bitcoin-core-nightly`](https://github.com/hebasto/bitcoin-core-nightly) repository: https://github.com/hebasto/bitcoin-core-nightly/actions/runs/12415868523/job/34663207030

ACKs for top commit:
  maflcko:
    re-ACK d9d5bc2
  achow101:
    ACK d9d5bc2
  mzumsande:
    Code Review ACK d9d5bc2
  tdb3:
    tested ACK d9d5bc2

Tree-SHA512: ad02adce98ce609176c9688289a0ca347932da5b6f259c6ab4e686914352f3d61f819adc150be80220f969200ee6a0c1c8737426525816fa0df58d688c51410c

test/functional/test_framework/util.py

@@ -453,6 +453,17 @@ def write_config(config_path, *, n, chain, extra_config="", disable_autoconnect=
         f.write("unsafesqlitesync=1\n")
         if disable_autoconnect:
             f.write("connect=0\n")
+        # Limit max connections to mitigate test failures on some systems caused by the warning:
+        # "Warning: Reducing -maxconnections from <...> to <...> due to system limitations".
+        # The value is calculated as follows:
+        #  available_fds = 256          // Same as FD_SETSIZE on NetBSD.
+        #  MIN_CORE_FDS = 151           // Number of file descriptors required for core functionality.
+        #  MAX_ADDNODE_CONNECTIONS = 8  // Maximum number of -addnode outgoing nodes.
+        #  nBind == 3                   // Maximum number of bound interfaces used in a test.
+        #
+        #  min_required_fds = MIN_CORE_FDS + MAX_ADDNODE_CONNECTIONS + nBind = 151 + 8 + 3 = 162;
+        #  nMaxConnections = available_fds - min_required_fds = 256 - 161 = 94;
+        f.write("maxconnections=94\n")
         f.write(extra_config)